In projection screens, in general, a Fresnel lens is used in combination with a lenticular lens. In order to maximally develop the optical effect, in use, the Fresnel lens and the lenticular lens are disposed so as to face each other and are brought into intimate contact with each other. The Fresnel lens functions to collimate projected light for correction in such a manner that the light is perpendicular to the screen face. On the other hand, the lenticular lens functions to diffuse light collimated by Fresnel lens mainly in a horizontal direction. In general, in the above projection screen, in use, the light outgoing surface side of the Fresnel lens (circular Fresnel convex lens) and the light incident surface side of the lenticular lens are brought into intimate contact with each other.
When lens faces of optical elements are brought into intimate contact with each other, since the surface of both the lenses has convexes and concaves, the surface shapes affect mutual surface shapes. For example, in the above example, the section of the Fresnel lens surface has a concave-convex shape with a sharp front edge like a cutting blade. On the other hand, the section of the lenticular lens surface is in a semicircular or semielliptic or other rounded protuberant arched concave-convex shape. When the Fresnel lens sheet and lenticular lens sheet having such respective sectional forms are brought into intimate contact with each other, the protuberant apex in the lenticular lens comes into contact with the sharp front edge in the Fresnel lens and the shape of the lenticular lens and/or the shape of the Fresnel lens, that is, the concave-convex shape on the lens surface, are deformed by contact pressure, resulting in lens collapse.
In these lens sheets, warpage is imparted to one lens sheet as for the lenticular lens and the Fresnel lens to come into intimate contact with each other, and both the lens sheets are brought to intimate contact with each other by taking advantage of the elasticity. In general, thermoplastic resins free from optical anisotropy are used as a polymeric resin material constituting the lenticular lens. Specifically, in order to improve impact resistance, a resin produced by copolymerizing butadiene rubber (BR) with polymethyl methacrylate (PMMA), polycarbonate (PC) or the like has been used. The thermoplastic resin with BR introduced thereinto sometimes causes warpage in the lens sheet due to moisture absorption of the resin or the like or causes defects such as waving or lenticulation of the lens due to a temperature change during extrusion.
In order to solve this problem, Japanese Patent Laid-Open Nos. 72848/1999 and 13388/2001 propose a lenticular lens comprising two materials, different from each other in coefficient of linear expansion or water absorption, which have been composited with each other. Japanese Patent Laid-Open No. 133886/2001 described above proposes such a construction that a multilayer construction is adopted in a lenticular lens sheet, and, in order to prevent the occurrence of local warp spots of the lens sheet, the addition amount of rubber particles in the layer (light incident side) which comes into contact with the Fresnel lens is reduced and the addition amount of rubber particles in the opposite side (light outgoing side) layer is increased. Further, Japanese Patent Laid-Open No. 72848/1999 proposes that a lenticular lens sheet having a multilayer construction is adopted and, in order to prevent the occurrence of lifting or warpage of the screen set attributable to a change in external humidity, PMMA or the like is used on the inner layer side (light incident side) of the lens sheet, while a polymethyl methacrylate-polystyrene copolymer having a low level of hygroscopicity is used on the outer layer side (light outgoing side).
In the construction described in Japanese Patent Laid-Open No. 133886/2001, in order to avoid vibration friction during transportation and the like, preferably, a larger amount of rubber particles are present on the surface of contact with the Fresnel lens. Even when the addition amount of rubber is increased, the friction resistance of the lens cannot be improved.
Further, in general, a resin which has been copolymerized with BR for enhancing the impact resistance of the lens is used. Accordingly, in the lens sheet described in Japanese Patent Laid-Open No. 72848/1999, when the content of BR is high, the hygroscopicity of the light incident side lens material becomes disadvantageously high and, consequently, the warpage level of the lens sheet per se is increased. In order to suppress the warpage of the lens caused by moisture absorption, the content of polystyrene (PS) should be increased to some extent. When the content of PS in the light incident side lens layer is enhanced, the impact resistance and friction resistance of the lens are disadvantageously deteriorated.
Further, in recent years, the development of a Fresnel lens formed of an ultraviolet curing resin which can be shaped into a high-definition lens form has been forwarded. When a lenticular lens formed of a thermoplastic resin is used in combination with a Fresnel lens formed of this ultraviolet curing resin, deformation or collapse of the lens caused by static contact force (i.e., pressure of contact between lenses) can be suppressed. However, the resistance to friction of the lenticular lens caused by dynamic contact force (i.e., dynamic contract friction of the lens caused during lens sheet transportation) is unsatisfactory.
Specifically, in an ultraviolet curing resin having a dense network structure in the polymer chain, the friction of lens by dynamic contact force can be suppressed by regulating the modulus of elasticity of the resin. In a thermoplastic resin constituted by entanglement of a polymer chain, in order to improve the friction resistance of the lens, a material design should be carried out from a viewpoint different from the modulus of elasticity.